I'm tinkering around with my own mode and I'm concerned that
some old remaining definitions might interfere when loading a
modified mode file. So I'd prefer to start with a clean slate
(without quitting Emacs every time).

you can clear out all the functions and variables whose names
begin w/ a prefix using `mapatoms' with a function that recognizes
those names and makes their definitions void via `fmakunbound' and
`makunbound', respectively.

If Dieter's mode properly `provide's its own feature, M-x unload-feature
should do the trick.
--
Kevin Rodgers